Studies being continued: 1) Colorado tick fever virus development in Dermacentor andersoni ticks, 2) comparative fine structure of Wolbachia-like organisms in D. andersoni, Rhipicephalus sanguineus and Haemaphysalis leporispalustris ticks, 3) comparative fine structure of Rickettsia rickettsii and Rickettsia sp. from Rhipicephalus sanguineus ticks from Mississippi in Vero cells, and 4) spiroplasma-like 277-F agent from H. leporispalustris-ticks. New investigations include: 1) developmental fine structure of the gut from normal and Rickettsia rickettsii infected D. andersoni ticks.